1. Field of Invention
The present invention relates generally to methods and apparatus for polishing the surface of a semiconductor wafer using a chemical mechanical polishing process. More particularly, the present invention relates to methods and apparatus for applying pressure differentials through a polishing pad to improve the performance of chemical mechanical polishing processes.
2. Description of Relevant Art
Chemical mechanical polishing, which is often referred to as "CMP," typically involves mounting a wafer, faced down, on a holder and rotating the wafer face against a polishing pad mounted on a platen. The platen is generally either rotating or in an orbital state. A slurry containing a chemical that chemically interacts with the facing wafer surface layer and an abrasive that physically removes portions of the surface layer is flowed between the wafer and the polishing pad, or on the pad in the vicinity of the wafer.
In semiconductor wafer fabrication, CMP is often utilized in an effort to planarize various wafer layers which may include layers such as dielectric layers and metallization layers. The planarity of the wafer layers is crucial for many reasons. For example, during wafer fabrication, planar layers reduce the likelihood of the accidental coupling of active conductive traces between different metallization layers, e.g., layers of active conductive traces, on integrated circuits housed on the wafer. Planar layers further provide a surface with a constant height for any subsequent lithography processes.
Polishing pressure, or the pressure applied to a wafer by a polishing pad, is generally maintained at a constant, e.g., uniform, level across the wafer. A uniform polishing pressure is maintained in an effort to ensure that the same amount of material, or film, is removed from all sections on the surface of a wafer. The amount of material removed from the surface of a wafer is governed by Preston's Equation, which states that the amount of material removed from the surface of a wafer is proportional to the product of polishing pressure and the relative velocity of the wafer. The relative velocity of the wafer is generally a function of the rotation of the wafer. Using Preston's Equation, if the relative velocity of the wafer is maintained at a constant level, and the polishing pressure is at a uniform level across the wafer, then the amount of material removed from the wafer is constant.
In general, many methods may be used to maintain a uniform polishing pressure. One method which is often used to maintain a uniform polishing pressure involves the use of an air bladder. FIG. 1 shows a cross-sectional representation of a chemical mechanical polishing pad sub-assembly 100 which may be used with a CMP apparatus such as an Avantgaard 676, available commercially from Integrated Processing Equipment Corporation (IPEC) of Phoenix, Ariz. A polishing pad 104 adheres to a flexible platen, or pad backer 108. An air bladder 112, which is arranged to inflate and deflate, is disposed between a plumbing reservoir 118 and pad backer 108.
Inflating air bladder 112, which is generally fabricated from an elastic material, effectively serves to apply a uniform pressure to polishing pad 104 and, hence, a wafer which is being polished using polishing pad sub-assembly 100. In general, air bladder 112 is arranged to essentially press against approximately the entire bottom surface of air bladder 112.
In an apparatus such as the Avantgaard 676, a slurry mesh 120 may be arranged between pad backer 108 and air bladder 112 as part of a system which is used to deliver slurry to the surface of polishing pad 104. Air bladder 112 then serves the additional purpose of pressurizing slurry mesh 120 to deliver slurry to the surface of polishing pad 104.
Although applying a uniform pressure to a polishing pad is generally effective to create a planar wafer surface, once the polishing pad has been repeatedly used, e.g., is near the end of its pad life, the effectiveness of the polishing pad decreases. That is, maintaining a uniform polishing pressure is not as effective in planarizing the wafer surface. Since replacing polishing pads is time-consuming and expensive, a polishing pad is typically repeatedly used until nonuniformity on the surfaces of wafers polished using the polishing pad is at a level which is considered to be unacceptable. Generally, after a polishing pad has been repeatedly used to polish wafers over a period of time, the polishing pad has a tendency to become "glazed." As is well known in the art, pad glazing occurs when the particles eroded from wafer surfaces, in addition to particles from abrasives in the slurry, glaze or otherwise accumulate over the polishing pad.
Pad glazing is generally most evident during CMP performed on an oxide layer such as a silicon dioxide layer. Herein and after, CMP performed on an oxide layer will be referred to as "oxide CMP." During oxide CMP, eroded silicon dioxide particulate residue, along with abrasives in the slurry, have the tendency to glaze the polishing pad. When pad glazing occurs, the polishing rate of the wafer surface is reduced, and a non-uniformly polished wafer surface is produced due to uneven removal of the glaze. By way of example, the peripheral region of the wafer surface may not be polished to the same extent as the center region of the wafer surface, due to conditions such as center-fast polishing and center-slow polishing. Center-fast polishing occurs when the material removal rate near the center, i.e., axial center, of a wafer surface is higher than the material removal rate away from the center of the wafer, whereas center-slow polishing occurs when the material removal rate near the center of a wafer surface is slower than the material removal rate away from the center of the wafer surface, as is well-known to those skilled in the art.
As such, what is desired is a method and apparatus for reducing wafer surface non-uniformity that occurs during CMP after a polishing pad has been used repeatedly. In other words, what is desired is a method and apparatus which enables the effectiveness of a polishing pad to be prolonged.